Dinosaurs Hunters

Your nine-year-old nephew draws a picture of a plesiosaur, which is a large, extinct marine reptile, some of which had long necks and well-developed fins. This plesiosaur is accurately depicted as swimming in an ocean, and in the sky above are a few pterosaurs, which were flying reptiles. One of the pterosaurs, however, is carrying a cow in its claws. Your nephew patiently explains to you that the "dinosaur" in the water is like the Loch Ness monster, and the "dinosaurs" flying overhead saw some cows in a field. One of them was hungry and wanted to feed its babies, so it captured the cow and was carrying it off to its nest.

How do you explain to him, without crushing his imagination or ego, some of the scientific inaccuracies of what he has illustrated and told you?